Orthogonal signal division multiple access for multiuser underwater acoustic networks

Abstract

Various embodiments comprise systems, methods, architectures, mechanisms and apparatus for optical. Radio Frequency (RF), and / or acoustic data transmission or communication by channel coding and interleaving a plurality of original bitstreams b0 — bN_1 to provide respective sequences of interleaved channel coded bits, which are respectively spatially modulated (SM) to provide respective symbol streams, each of which is divided into a plurality of blocks wherein each block has inserted thereat a pilot sequence associated with the relevant symbol stream, each block and its inserted pilot sequence being OSDM modulated, the OSDM modulated blocks being multiplexed for transmission via optical sources, RF antennae, acoustic transducers, and the like during respective limeslots such that a receiver associated with a plurality of receiving devices may reconstruct the original bitstreams b0 —bN_1.

Description

ORTHOGONAL SIGNAL DIVISION MULTIPLE ACCESS FOR MULTIUSER UNDERWATER ACOUSTIC NETWORKS CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims the benefit of US Provisional Patent Application Serial No.63 / 540,089 filed September 24, 2023, which Application is incorporated herein by reference in its entirety. GOVERNMENT INTEREST

[0002] This invention was made with government support under NSF NeTS Award No. CNS-1763964. The government has certain rights in the invention. FIELD OF THE DISCLOSURE

[0003] The present disclosure generally relates to communications and, in particular, to Orthogonal Signal Division Multiple Access (OSDMA) communications systems. BACKGROUND

[0004] The extreme and uncertain underwater environment presents significant challenges for achieving reliable acoustic communications, mainly due to limited bandwidth and time-varying channels. With the increasing demand for reliable underwater / undersea wireless communication systems, three main-stream communications mechanisms are typically considered: Underwater Radio Frequency Communication (URFC), Underwater Acoustic Communication (UAC), and Underwater Wireless Optical Communication (UWOC).

[0005] URFC provides high bandwidth and data rate, but radio waves suffer high attenuation in seawater due to the high conductivity and permittivity, leading to a limited coverage distance of up to a few meters. UWOC offers a wider bandwidth on the scale of hundreds of megahertz. However, the UWOC suffers from water absorption and scattering effects and attenuates greatly. Additionally, some alignments between the transmitter and the receiver are required, and the quality of the communication link can be severely impaired by external factors, such as the presence of sources of reflection, e.g., bubbles.

[0006] Different from URFC and UWOC, UAC suffers less attenuation and covers a communication range of up to kilometers, but the underwater acoustic speed is as slow as 1500 m / s, leading to time-varying multipath delay. Moreover, the Doppler effects caused bythe dynamic water wave, low bandwidth, sound speed variability, and frequency-dependent scattering losses still make UAC a challenge.

[0007] Improvements are desired. SUMMARY

[0008] Various deficiencies in the prior art are addressed by systems, methods, architectures, mechanisms, and apparatus for optical, Radio Frequency (RF), and / or acoustic data transmission or communication by channel coding and interleaving a plurality of original bitstreams ^^^െ ^^ேି^to provide respective sequences of interleaved channel coded bits, which are Spatially Modulated (SM) to provide respective symbol streams, eachof which is a plurality of blocks wherein each block has inserted thereat a pilot sequence associated with the relevant symbol stream, each block and its inserted pilot sequence being OSDM modulated, the OSDM modulated blocks being multiplexed for transmission via optical sources, RF antennae, acoustic transducers, and the like during respective timeslots such that a receiver associated with a plurality of receiving devices may reconstruct the original bitstreams ^^^െ ^^ேି^.

[0009] A method of communication according to an embodiment comprises, at a transmitter: channel coding and interleaving each of a plurality of input data streams to provide respective sequences of channel coded bitstreams; Spatially Modulating (SM) and mapping to a constellation each of the respective sequences of channel coded bitstreams to provide respective sequences of symbol streams ^^^; dividing each of the sequences of symbol streams ^^^into ^^^blocks by serial-to-parallel conversion; modulating each of the ^^^symbol stream blocks and a respective pilot sequence in accordance with Orthogonal Signal Division Multiplexing (OSDM) to provide thereby respective OSDM signals; and transmitting the OSDM signals via a transmitting device during respective time slots. The communication method may comprise an undersea communication method, and each respective transmitting device a submerged acoustic output device. The communication method may comprise a Radio Frequency (RF) communication method, and each respective transmitting device a RF antenna.

[0010] The method of communication may also comprise, at a receiver: receiving, via each of a plurality of acoustic input devices, some or all of the transmitted OSDM signals; channel estimating received OSDM signals using pilot sequences included therein; channel equalizing and OSDM demodulating received OSDM signals to provide aplurality of channel equalized symbol stream blocks; combining respective ^^^blocks by parallel-to-serial conversion into sequences of symbol streams ^^^; spatially demodulating and constellation de-mapping the channel equalized received sequences of symbol streams ^^^to retrieve therefrom respective sequences of channel coded bitstreams; and de-interleaving and channel decoding the retrieved sequences of channel coded bitstreams to obtain therefrom respective output bit sequences.

[0011] Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows and will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, explain the principles of the present invention.

[0013] FIGS.1A-1B illustrate the symbol arrangement of two-user Orthogonal Frequency Division Multiple Access (OFDMA) and Orthogonal Signal Division Multiple Access (OSDMA) systems, respectively, in the time and frequency domains;

[0014] FIG.2 schematically depicts OSDMA modulation for user # ^^ with pilot sequences;

[0015] FIG.3 schematically depicts an OSDMA system according to an embodiment;

[0016] FIG.4 schematically depicts a high-level structure of a physical-layer emulator useful in explaining the various embodiments;

[0017] FIG.5 depicts a high-level block diagram of a controller suitable for use in various embodiments, such as to implement a transmitter, receiver, transceiver, system, or component thereof in accordance with the various embodiments;

[0018] FIGS.6A-6C graphically illustrate channel impulse response, power spectrum of channel response, and power spectrum density of transmitted / received signals, respectively, collected base-band channel information from two locations;

[0019] FIGS.7A-7F graphically illustrate with two users BER as a function of normalized SNR (Eb / No) with different Data-to-Pilot Ratio (DPR, ^^^^) under different conditions;

[0020] FIGS.8A-8C graphically illustrate the performance of two-user OSDMA / OFDMA ( ^^ ൌ 2);

[0021] FIGS.9A-9C graphically illustrate the BER of OSDMA and OFDMA when the water wave speed is 0.3 m / s;

[0022] FIG.10 illustrates an Orthogonal Frequency Division Multiplexing (OFDM) frame structure suitable for use in various embodiments;

[0023] FIG.11 depicts a flow diagram of a synchronization method according to an embodiment and suitable for implementing a synchronization strategy at a receiver; and

[0024] FIG.12 schematically depicts an OSDMA system according to an embodiment.

[0025] It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the sequence of operations as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes of various illustrated components, will be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments have been enlarged or distorted relative to others to facilitate visualization and clear understanding. In particular, thin features may be thickened, for example, for clarity or illustration. DETAILED DESCRIPTION

[0026] The following description and drawings merely illustrate the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art and are to be construed as being without limitation to such specifically recited examples and conditions. Additionally, the term, "or" as used herein, refers to a non-exclusive or, unless otherwise indicated (e.g., “or else” or “or in the alternative”). Also, the various embodiments described herein are not necessarily mutually exclusive, as someembodiments may be combined with one or more other embodiments to form new embodiments.

[0027] The numerous innovative teachings of the present application will be described with particular reference to the presently preferred exemplary embodiments. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. Those skilled in the art and informed by the teachings herein will realize that the invention is also applicable to various other technical areas or embodiments.

[0028] Various systems, methods, architectures, mechanisms, and apparatus are disclosed herein for optical, Radio Frequency (RF), and / or acoustic data transmission or communication by channel coding and interleaving a plurality of original bitstreams ^^^െ ^^ேି^to provide respective sequences of interleaved channel coded bits, which are respectively spatially modulated (SM) to provide respective symbol streams, each of which is divided into a plurality of blocks wherein each block has inserted thereat a pilot sequence associated with the relevant symbol stream, each block and its inserted pilot sequence being OSDM modulated, the OSDM modulated blocks being multiplexed for transmission via optical sources, RF antennae, acoustic transducers, and the like during respective timeslots such that a receiver associated with a plurality of receiving devices may reconstruct the original bitstreams ^^^െ ^^ேି^.

[0029] The inventors have applied Phase-Shift Keying (PSK)-based OFDM to underwater environments. For example, to estimate the time-varying UAC channels accurately, the inventors have proposed PSK-based OFDM systems in which a pilot sequence is inserted behind every data sequence. While improving channel estimation, this arrangement exhibits a reduction in spectral efficiency. The inventors have also introduced the usage of Kronecker product with rows of Discrete Fourier Transform (DFT) matrix, which shows better Bit Error Rate (BER) performance compared with the traditional OFDM method under the heavy multipath environment.

[0030] The inventors have also proposed Orthogonal Signal Division Multiplexing (OSDM) for UAC, which achieves lower Peak-to-Average Power Ratio (PAPR) and higher robustness against the Doppler effect. Zhuoran Qi and Dario Pompili. UW-CTSM: “CircularTime Shift Modulation for Underwater Acoustic Communications” in 17th Wireless On- Demand Network Systems and Services Conference (WONS), pages 1–8, 2022.

[0031] To improve the transmission data rate and reduce the inter-channel interference, the inventors proposed Spatial Modulation (SM)-based OSDM in, where transducers are coded and only one of them is active in each time slot. Zhuoran Qi and Dario Pompili: “Spatial Modulation-Based Orthogonal Signal Division Multiplexing for Underwater ACOMMS” in Underwater Communications and Networking (UComms), pages 1–5, 2022

[0032] Various embodiments provide a novel multiple-access technique denoted herein as Orthogonal Signal Division Multiple Access (OSDMA) suitable for use at the physical layer, which is feasible for UAC systems. OSDMA enables the simultaneous transmissions of different data streams from a single transmitter to corresponding receivers / users by modulating the streams with an orthogonal Inverse Discrete Fourier Transform (IDFT) matrix. In a low-noise environment, each receiver / user can accurately estimate the channel and demodulate the data. Emulations and simulations based on collected at-sea channel state information demonstrate that OSDMA offers higher robustness against underwater time- varying channels than traditional Orthogonal Frequency Division Multiple Access (OFDMA).

[0033] CDMA and OFDMA show high spectrum efficiency and have been widely used. CDMA allocates orthogonal codes to different users, allowing them to transmit signals in the same frequency band and time slot. This technique is inherently robust to frequency-selective fading and compensates for the effects of multipath at the receiver. However, the spectral efficiency of CDMA is relatively low when the number of users is small. In OFDMA, the frequency band is divided into several orthogonal subcarriers. These subcarriers are allocated to different users, so multiple users can share the same frequency band and transceiver data streams simultaneously. Since the subcarriers are orthogonal to each other, the data streams do not interfere with each other. However, when used in UAC, the orthogonality of subcarriers is disturbed by the dynamic water environment, which leads to Inter-Symbol Interference (ISI) and Inter-Carrier Interference (ICI) caused by multipath and Doppler effects. As a result, the performance of OFDMA is significantly degraded in UAC scenarios. Moreover, the high PAPR in OFDMA can also cause distortions in linear devices, such as power amplifiers. Therefore, there is an urgent need to develop a new technique that can overcome these challenges and ensure reliable UAC networks.

[0034] Various embodiments provide systems, apparatus, and methods enabling multiuser downlink UAC networks, called OSDMA, to defend against time-varying multipath and Doppler effects in the underwater acoustic environment. The underlying techniques described herein achieve multiple access through OSDM for reliable UAC networks. The derived equations of the modulator and demodulator of OSDMA advantageously reduce the ISI and ICI at the physical layer. The transceiver structure of the OSDMA is designed, and the modulation / demodulation process is also demonstrated in detail with block diagrams.

[0035] The physical-layer evaluations for two-user OSDMA system is carried out by MATLAB based on collected channels from at-sea experiments at Barnegat Bay, NJ, in October 2022. The BER performances are presented and analyzed. The proposed OSDMA considers different water wave speeds (the moving speeds of the propagation medium), varying Signal-to-Noise Ratio (SNR) values, and different pilot and data sequence ratios. The results of OSDMA and OFDMA are compared and analyzed, which present the advantages of OSDMA over OFDMA in UAC networks. The trade-off between OSDMA and OFDMA is also discussed. The OSDMA spectral efficiency versus resource allocation to different numbers of users is simulated and analyzed by considering different values of the fairness index.

[0036] FIGS.1A-1B illustrate the symbol arrangement of two-user Orthogonal Frequency Division Multiple Access (OFDMA) and Orthogonal Signal Division Multiple Access (OSDMA) systems, respectively, in the time and frequency domains. Unlike OFDMA, where symbols are spread only in the frequency domain, OSDMA spreads symbols in both the time and frequency domains. This feature makes OSDMA particularly suitable for the underwater environment, where the multipath and Doppler effects result in significant ISI and ICI. Specifically, FIGS.1A-1B depict the time-frequency structure of communication signals of two users, with Cyclic Prefix (CP) inserted. In FIG.1A, OFDMA is used and each symbol is spread in the time domain and occupies one subcarrier, whereas in FIG.1B OSDMA is used and each symbol is spread in both the time and frequency domains.

[0037] FIG.2 schematically depicts OSDMA modulation for user # ^^ with pilot sequences. Specifically, the OSDMA modulation depicted in FIG.2 is useful for illustrating Equations (2) and (6) as discussed in more detail below. Whereas ^^^is the symbol sequence for user # ^^, ^^^is the IDFT submatrix, ^^ఈು^is one of the rows in ^^^, ^^୬is the OSDM modulated symbol sequence, “⊗” denotes Kronecker product and “Σ” denotes summation.

[0038] FIG.3 schematically depicts an OSDMA system according to an embodiment. Specifically, the exemplary OSDMA system 300 of FIG.3 comprises a modulator 310 providing output signal to a transmitter 320, which transmits signal to a plurality of receivers 330, which provides received signal to a demodulator 340.

[0039] Assuming there is one transmitter 320 and ^^ receivers / users 330, and there is one transducer at the transmitter and one hydrophone at each receiver. ^^^represents the symbol sequence for ^^-th user (user # ^^), ^^ ൌ 0,1, ... , ^^ െ 1. The OSDMA modulator (310) takes the symbol sequence ^^^for user # ^^, which has already been modulated and had pilot sequence inserted and performs a Kronecker product with the Inverse Discrete Fourier Transform (IDFT) submatrix ^^^to spread ^^^in the time and frequency domains. This result is then transmitted acoustically via a hydrophone (“Tx") after a Cyclic Prefix (CP) has been inserted. At user # ^^, the transmitted stream is received, and the channel is estimated based on the unique channel ^^^^^after removing the CP (342). Each user’s symbols can be recovered at the OSDMA demodulator (340) by performing a Kronecker product with ^^^ு, which is the conjugate transpose of ^^^. The details of OSDMA modulation and demodulation are as follows.

[0040] Transmitter Discussion: To modulate each symbol sequence with OSDM, ^^^is divided to ^^^blocks by serial-to-parallel conversion, ^^^ൌ ^ ^^^,^^^^,ଶ... ^^^,^^^. (1)

[0041] Each block ^^^,^has a length of ^^, ^^ ൌ 1, ... , ^^^, the length of ^^^is ^^^^^.

[0042] To estimate the channel characteristics, the pilot sequence ^^^,^is added in front of ^^^, then the symbol vector ^^^with the length of ^^^^^ is created, ^^^ൌ ^^^^ 1, ^^^ൌ ^ ^^^,^^^^,^^^^,ଶ... ^^^,^^^. (2)

[0043] Note that only onepilot sequence can be shared among all OSDMA users for channel estimation at their respective receivers. If no pilot sequence is added to user # ^^, ^^^ൌ ^^^, ^^^ൌ ^^^.

[0044] Let ^^ ^ ∑^^^^ൌ ∑^^^^^ 1, ^^ is an even number, ^^ is the IDFT matrix of size ^^ ൈ ^^,^^^^^^^^⋯ ^^^^é^^^ ^ ^ି^ ù é ^^^ù ^ê^ ^^^ ⋯ ^^^úê^^^ú (3)(4) whereare and have unit magnitude, i.e., ^^^^^ு^ ൌ 1, ^^^^^ு^ᇱൌ 0for ^^ ് the transpose conjugate of a matrix.

[0045] Assuming a finite set Ω of size ^^, Ω ൌ ^0,1,2, … , ^^ െ 1^. Let Ω^, Ω^, … , Ωேି^be ^^ disjoint subsets of Ω, such that each element ^^ ∈ Ω belongs to exactly one of the subsets Ω^. Furthermore, no two subsets have any elements in common. In other words, ⋃ே^ୀି^^Ω^ൌ Ω, and Ω^∩ Ω^ᇱൌ ⌀ for ^^ ് ^^′.

[0046] For ^^-th user, the IDFT submatrix of size ^^^ൈ ^^ is given by, ^^ éఈబ^^ ù(5)

[0047] Based on OSDM modulation, the transmitted stream for user # ^^ is multiplexed into one stream ^^^of length ^^ ^^, as shown in FIG.2, ^^^ൌ ^^^^ ^^^⊗ ^^ொ^, (6) where “⊗” denotes^^ ൈwords, the rows of ^^^are formed by rows of ^^ in (5).

[0048] Finally, the transmitted signal at the transmitter is expressed by, ^^ ൌ ∑^^^^ൌ ^ ^^^^^ଶ⋯ ^^^ொି^^. (7)

[0049] In this manner, theon a rectangular lattice in the time and frequency domains and do not interfere with each other.

[0050] To reduce the ISI, a CP with a length of ^^ is added,^ ^^ ൌ ^ ^^^ொି^^^^ொି^ା^⋯ ^^^ொି^^^^, (8) where ^^ ^ ^^. Assume ^^௧is the maximum multipath delay in the time-discrete model. When ^^ ^ ^^௧, the channel reverberation is correctable.

[0051] To evaluate the fairness of system resource allocation, we consider Jain’s fairness index, మ ^^^ൌ^∑^ ^^^^ே^ೌ^^∑^ ^^మ^ ^, (9) where ^^^is thefor the pilot sequence, ^^^^௫ൌ ^^ െ 1.

[0052] Receiver Discussion: With the orthogonality of ^^^, ^^^,^can be recovered from ^^^by the receiver as follows, ^^^^ ^^ுఈ^⊗ ^^ொ^ ൌ ^^^^ ^^^⊗ ^^ொ^^ ^^ுఈ^⊗ ^^ொ^where ^^^ொൈொand ^^^^^ି^^ொൈொare zero matrices.

[0053] Further, ^^^can be recovered from ^^ by the receiver as follows, ^^^ ^^^ு⊗ ^^ொ^ ൌ ^^^^ ^^^ு⊗ ^^ொ^ ^∑^ᇱஷ^ ^^^ᇱ^ ^^^ு⊗ ^^ொ^

[0054] ^^^,^ൌ ^^ுఈ^⊗ ^^ொ, (12a)ൌ

[0055] Assume ^^^^^ൌ ^ℎ^^^ ^^^^ ℎ^... ^ is the channel response at user # ^^, the received signal is,^^^^ൌ ^ ^^ ∗ ^^^^^, (13) where “∗” denotes the convolution operation. The CP-removed sequence ^^^^canby,^^^ൌ ^^ ^^^, ℎ^^^ ℎ^^^ ⋯ ^^^é^ ^ℎ^ொି^ù )

[0056] Given ^^ ^ ^^ ^can be expressed as: ^^^ℎ^^^é^ℎ^⋯ ℎொି^0 ⋯ 0ù ⋯^^^⋯

[0057] The^^^^^^,^ൌ ^^^,^^^^,^, (16a) wherethe transmission.

[0058] Let ^^^ൌ 0, we get the relationship between ^^^and the pilot sequence ^^^,^, ℎ^^^ ℎ^^^ ⋯ ^^^é^ ^ℎொି^ù

[0059] The high-autocorrelation pilot sequence ^^^,^is created with a length of ^^. The elements in ^^^,^can be expressed by: మ ^^^,^^ ^^^ ൌ exp^ଶగ^^ଶொ ^, (18)where ^^ ൌ√െ1, ^^ ൌsignal, whose periodic auto-correlation function is an impulse

[0012] , i.e., ^ ொ^^^,^^^^ு,^ൌ ^1 0 ⋯ 0^, (19a)é^^^,^^0^ ^^^,^^1^ ⋯ ^^^,^^ ^^ െ 1^^^ ^ ^^ െ 1^ ^^ ^0^ ⋯ ^^ ^ ^^ù ^,^ ^,^ ^,^ െ 2^^^that we can estimate the channel impulse response by calculating the periodic cross-correlation function between ^^^and ^^^,^, ^ ொ ^^ ^^ ^^ு ^^^ ^^^ ^^^ ^^^^^,^ ^,^ൌ ^^^^௧ൌ^ℎ^ℎ^⋯ ℎொି^ ൧. (20)

[0060] With thecan demodulate the OSDM signal by, ^^̂^ ൌ ^^^ ^^^^ு^ ^^^^ ^^^^ு^ି^^ ^^^ு⊗ ^^ொ^. (21)

[0061] After removing the

[0062] Thus, referring to FIGS.1-4, a method of communication according to an embodiment comprises, at a transmitter: channel coding and interleaving each of a plurality of input data streams to provide respective sequences of channel coded bitstreams; Spatially Modulating (SM) and mapping to a constellation each of the respective sequences of channel coded bitstreams to provide respective sequences of symbol streams ^^^; dividing each of the sequences of symbol streams ^^^into ^^^blocks by serial-to-parallel conversion; modulating each of the ^^^symbol stream blocks and a respective pilot sequence in accordance with Orthogonal Signal Division Multiplexing (OSDM) to provide thereby respective OSDM signals; and transmitting the OSDM signals via a transmitting device during respective time slots. The communication method may comprise an undersea communication method, andeach respective transmitting device a submerged acoustic output device. The communication method may comprise a Radio Frequency (RF) communication method, and each respective transmitting device a RF antenna.

[0063] The method of communication may also comprise, at a receiver: receiving, via each of a plurality of acoustic input devices, some or all of the transmitted OSDM signals; channel estimating received OSDM signals using pilot sequences included therein; channel equalizing and OSDM demodulating received OSDM signals to provide a plurality of channel equalized symbol stream blocks; combining respective ^^^blocks by parallel-to-serial conversion into sequences of symbol streams ^^^; spatially demodulating and constellation de- mapping the channel equalized received sequences of symbol streams ^^^to retrieve therefrom respective sequences of channel coded bitstreams; and de-interleaving and channel decoding the retrieved sequences of channel coded bitstreams to obtain therefrom respective output bit sequences.

[0064] FIG.5 depicts a high-level block diagram of a controller suitable for use in various embodiments, such as to implement a transmitter, receiver, transceiver, system, or component thereof in accordance with the various embodiments. The controller 505 depicted in FIG.5 comprises a computing device that may be configured to perform various computing, processing, control, and / or other functions such as described herein with respect to the figures and equations. For example, the controller 505 may perform various transmitter and / or receiver functions such as described herein with respect to the various figures and equations.

[0065] As depicted in FIG.5, the controller 505 includes one or more processors 510, a memory 520, a communications interface 530, and input-output (I / O) interface(s) 540. The processor(s) 510 are coupled to each of memory 520, communication interfaces 530, and I / O interfaces 540.

[0066] The processor(s) 510 are configured for controlling the operation of controller 505, including operations supporting the methodologies described herein with respect to the various embodiments. Similarly, the memory 520 is configured for storing information suitable for use by the processor(s) 510. Specifically, memory 520 may store programs 521, data 522 and so on. Within the context of the various embodiments, the programs 521 and data 522 may vary depending upon the specific functions implemented by the controller 505, such as transmitter functions 101, receiver functions 102, or transmitter and receiver (i.e., transceiver 101 / 102) functions.

[0067] For example, as depicted in FIG.5, the programs portion 521 of memory 520 includes a OSDMA modulator 521-MOD and other transmitter functions 521-OTF suitable for implementing transmitter functions 101 in accordance with various embodiments, as well as a OSDMA demodulator 521-DEMOD and other receiver functions 521-ORF suitable for implementing receiver functions 102 in accordance with various embodiments. Further, the programs portion 521 of memory 520 includes a computing, control, management, and / or other functions module 521-CCM suitable for implementing various computing, control, management, data processing, communications, and / or other functions suitable for use within the various embodiments, such as the various other primary or supporting functions discussed in this specification.

[0068] Generally speaking, the memory 520 may store any information suitable for use by the controller 505 in implementing one or more of the various methodologies or mechanisms described herein. It will be noted that while various functions are associated with specific programs or databases, there is no requirement that such functions be associated in a specific manner. Thus, any implementations achieving the functions of the various embodiments may be used.

[0069] The communications interfaces 530 may include one or more services signaling interfaces adapted to facilitate the transfer of information, files, data, messages, requests and the like between various entities in accordance with the embodiments discussed herein.

[0070] The I / O interface 540 and / or communications interfaces 530 may be coupled to receive input bitstreams for OSDMA modulation and transmission and / or to provide output bitstreams resulting from OSDMA demodulation. Various input / output devices may be used for such purposes.

[0071] Various embodiments are implemented using a controller 505 comprising processing resources (e.g., one or more servers, processors and / or virtualized processing elements or compute resources) and non-transitory memory resources (e.g., one or more storage devices, memories and / or virtualized memory elements or storage resources), wherein the processing resources are configured to execute software instructions stored in the non- transitory memory resources to implement thereby the various methods and processes described herein. As such, the various functions depicted and described herein may be implemented at the elements or portions thereof as hardware or a combination of software and hardware, such as by using a general-purpose computer, one or more Application Specific Integrated Circuits (ASIC), or any other hardware equivalents or combinationsthereof. In various embodiments, computer instructions associated with a function of an element or portion thereof are loaded into a respective memory and executed by a respective processor to implement the respective functions as discussed herein. Thus, various functions, elements and / or modules described herein, or portions thereof, may be implemented as a computer program product wherein computer instructions, when processed by a computing device, adapt the operation of the computing device such that the methods or techniques described herein are invoked or otherwise provided. Instructions for invoking the inventive methods may be stored in tangible and non-transitory computer readable medium such as fixed or removable media or memory or stored within a memory within a computing device operating according to the instructions.

[0072] It is contemplated that some of the steps discussed herein as software methods may be implemented within special-purpose hardware, for example, as circuitry that cooperates with the processor to perform various method steps.

[0073] Although primarily depicted and described as having specific types and arrangements of components, it will be appreciated that any other suitable types and / or arrangements of components may be used for controller 505.

[0074] This section provides a detailed overview of the physical-layer parameters of the OSDMA system. To demonstrate the performance of the OSDMA system, present herewith are emulated BER results for two-user OSDMA, under different SNR values and multipath delays. Also investigated are the impact of Doppler frequency shifts at various water wave speeds and the need for Doppler compensation. In addition, evaluation of multiuser OSDMA performance is provided via simulation results for resource allocation fairness and spectral efficiency.

[0075] FIG.4 schematically depicts a high-level structure of a physical layer structure useful in explaining the various embodiments. The structure 400 includes both a transmitter 401 and a receiver 402, each having a plurality of modules. S / P 415 is a serial-to-parallelconverter. ^^^ is the bit sequence for user # ^^ and ^^^ ^ is the decoded bit sequence for user # ^^,where ^^ ൌ 0,1, … , ^^ െ 1.

[0076] Referring to FIG.4, at the OSDMA system transmitter 401, the input bit sequences are received and processed in accordance with channel (e.g., turbo) coding and interleaving 405 (e.g., forward error control coding (FECC) and the like) before being modulated with Quadrature Phase Shift Keying (QPSK) via constellation mapping 410. The resulting coded bits are then converted from serial to parallel via S / P module 415 and pilotsequences are then inserted 420 for channel estimation. The symbol sequences are then modulated 430 and transmitted 435 using OSDMA. At each receiver (4400through 440N-1), the channels are estimated by analyzing the pilot sequences. The received signals are then subject to channel equalization and OSDMA demodulation 450. After removing the pilot sequences 460, the constellation symbols are obtained and subjected to constellation de- mapping 470 to get the demodulated bits. Finally, the output bits are obtained by de- interleaving and channel (e.g., turbo) decoding 480. This transceiver structure allows for efficient and reliable communication over the OSDMA system.

[0077] The emulations in this study are based on real UAC channels collected at Barnegat Bay, NJ, in October 2022 using USRP X-300 boards, with a depth of 0.5 m and a distance range of about 10 m. To ensure the statistical relevance of the emulation results, we provide 95% confidence intervals.

[0078] FIGS.6A-6C graphically illustrate channel impulse response, power spectrum of channel response, and power spectrum density of transmitted / received signals, respectively, collected base-band channel information from two locations. The bandwidth is 100 kHz, and the symbol rate is 100 kBd. The normalized SNR (Eb / No) is about 20 dB.

[0079] Assume that the locations of the transmitter and receivers are fixed, so the Doppler effect is mainly from the water wave. In the emulations, the wave speed is assumed to range in ^0.3, 0.5^ m / s. The source modulation order is ^^ ൌ 2 for QPSK. The turbo coding rate is ^^^^^ൌ 1 / 3. The emulated bandwidth is set to 100 kHz. The additive noise is assumed to be the Additive White Gaussian Noise (AWGN) with SNR varying from 5 dB to 20 dB. To assess the robustness of the OSDMA / OFDMA system against Doppler effects, we introduce the Data-to-Pilot Ratio (DPR, ^^^^). Since the Doppler frequency shift affects the channel coherence time (the channel is assumed not to vary during a coherence time), we use DPR to compare the performance of OSDMA / OFDMA systems under different Doppler conditions. MATLAB is utilized for signal processing, including coding / modulation at the transmitter and decoding / demodulation at the receivers.

[0080] A major finding of the emulation results includes system robustness enhancement when in channels with multipath delay and Doppler effects.

[0081] FIGS.7A-7F graphically illustrate with two users BER as a function of normalized SNR (Eb / No) with different Data-to-Pilot Ratio (DPR, ^^^^) under different conditions. When water wave speed is 0.3 m / s, FIG.7A depicts ^^௧ൌ 10, FIG.7B depicts^^௧ൌ 30, and FIG.7C depicts ^^௧ൌ 60. When water wave speed is 0.5 m / s, FIG.7D depicts ^^௧ൌ 10, FIG.7E depicts ^^௧ൌ 30, and FIG.7F depicts ^^௧ൌ 60. ^^௧is the maximum multipath delay in the time-discrete model. ^^^is the Jain fairness index. ^^ ൌ ^^^^^ 2 is the IDFT matrix size for OSDMA.

[0082] FIGS.8A-8C graphically illustrate the performance of two-user OSDMA / OFDMA ( ^^ ൌ 2). For OSDMA, when ^^^^ൌ 6, there is one pilot sequence and 6 data sequences. We allocate ^^^^ / ^^ ൌ 3 IDFT rows to each of the two users and leave one row idle. Given that K is an even number and ^^ ^ ^^^^^ 1, we let ^^ ൌ 8. According to the above equations (9), the maximum number of users is ^^^^௫ൌ ^^ െ 1 ൌ 7 and ^^^ൌ ^మ^ൈ^ଷమାଷమା^మ^ൌ 0.37. For OFDMA, there are 64 subcarriers in total, each user gets 32, so ^ସమ ^^^ൌ^ସൈ^ଷଶమାଷଶమ^ൌ 0.03. 9C graphically illustrate the BER of OSDMA and OFDMA when thewater wave speed is 0.3 m / s. It can be observed that the BER performances of OSDMA with different DPR values are very close to each other, since no matter what DPR is, the pilot sequences are always spread in every time slot. Moreover, we can find that even with a heavier multipath delay, the BER of OSDMA increases slightly, showing the robustness against multipath delay. We can also find that OFDMA with DPR of 2 has a lower BER than OSDMA, but when DPR is increased to 4 and 6, the OFDMA shows a higher BER than OSDMA, since the pilot sequence of OFDMA only spreads in subcarriers and cannot cover every time slot. When DPR is 4, the OSDMA reaches SNR gain of 6 dB compared with OFDMA when ^^௧ൌ 10. With a higher speed, the Doppler frequency shift increases, and the coherence time is shortened.

[0084] Comparing FIG.7A and FIG.7D, it can be seen that the coverage distance of OSDMA does not change much. In FIGS.7D-7F, it can be seen that the OFDMA fails to transmit signals correctly when DPR is 4 or 6. The reason is that the time interval between two pilot sequences is longer than the coherence time, so the channel cannot be estimated accurately.

[0085] Referring to FIGS.8A-8C and 9A-9C, multiuser OSDMA is simulated with different scenarios. With ^^ ൌ 8, ^^^^௫ൌ 7 and ^^^ൌ 1, the BER versus Eb / No with a different number of OSDMA users when water wave speed is 0.3 m / s: (FIG.8A) ^^௧ൌ 10; (FIG.8B) ^^௧ൌ 30; (FIG.8C) ^^௧ൌ 60. FIGS.8A-8C depict the simulated BER performanceof OSDMA with different numbers of users when ^^ ൌ 8 and ^^^ൌ 1, where each row of the IDFT matrix is assigned to one user. The maximum number of users is ^^^^௫ൌ ^^ െ 1 ൌ 7, ^మ and the fairness index is ^^^ൌ^ൈ^ൌ 1. When there are ^^ users, ^^ rows of the IDFT matrix are occupied, and ^^^^௫െ ^^ rows are idle. From FIGS.8A-8C, it can be seen that the BER with ^^ ൌ 2 is lower than ^^ ൌ 4 and ^^ ൌ 6, because there is less inter-channel interference.

[0086] FIGS.9A-9C illustrate the impact of ^^^and the number of users on the spectral efficiency in a multiuser OSDMA system when Eb / No is 20 dB, ^^௧ൌ 10, and the water wave speed is 0.3 m / s. When Eb / No ൌ 20 dB, ^^௧ൌ 10, and the water wave speed is 0.3 m / s, the spectral efficiency versus numbers of users with different fairness index: (FIG.9A) ^^ ൌ 12; (FIG.9B) ^^ ൌ 14; (FIG.9C) ^^ ൌ 16. Note that the maximum number of users is ^^ െ 1. When the number of users is small, increasing ^^^results in a decrease in spectral efficiency due to inefficient allocation of resources to inactive users. Comparing FIGS.9A-9C, it can be seen that when ^^^ൌ 1, the spectral efficiency decreases with increasing ^^ for a fixed and small number of users since larger ^^ allocates more resources to inactive users. The spectral efficiency cannot be improved beyond a certain point, e.g., as shown in FIG.9A, where ^^ ^ 8, since all the resources are allocated to active users. Moreover, a larger ^^ leads to a higher DPR and maximal spectral efficiency. Nonetheless, the overall spectral efficiency is lower than 0.7 due to the narrow bandwidth in UAC, low transmission data rate, and the need to use channel coding with low coding rate and source modulation order to ensure reliable transmissions.

[0087] The various embodiments provide a novel multiple-access technique at the physical layer, named Orthogonal Signal Division Multiple Access (OSDMA), to improve the multiuser system robustness against severe underwater acoustic environments. The OSDMA utilizes the orthogonal IDFT submatrices to spread each user symbol sequence to both the time and frequency domains. To evaluate the performances, we conducted two-user emulations and multiuser simulations based on real channels collected at Barnegat Bay. The results indicated that compared with traditional OFDMA, our proposed OSDMA had higher robustness against heavy multipath delay and high Doppler frequency shifts. The trade-off between OSDMA and OFDMA versus channel scenarios was discussed. The spectral efficiency versus resource allocation was also explored. Improved Time Synchronization

[0088] UAC faces significant challenges due to high latency and multipath delay; at times preventing the receiver from accurately determining the signal arrival time. Therefore, effective signal detection and time synchronization are crucial. Acoustic noise within UAC environments may cause the background noise power to fluctuate unpredictably. In addition, if the transmitter or receiver is in motion, the variation of distance causes changes in the received power and latency over time.

[0089] As such, various embodiments provide a time synchronization strategy for dynamic UAC scenarios. This strategy utilizes the autocorrelation of M-sequences and power detection to effectively address issues related to multipath delay, as well as varying noise and signal power at the receiver.

[0090] FIG.10 illustrates an Orthogonal Frequency Division Multiplexing (OFDM) frame structure suitable for use in various embodiments. Specifically, as can be seen in FIG. 10, a transmitted OFDM frame is composed of zero Guard Intervals (GIs), M-sequence, and OFDM data blocks. Each OFDM data block contains the Cyclic Prefix (CP), whose length is equal to the number of subcarriers so that the multipath can be fully captured. The zero GI is used to monitor background noise and reduce Inter-Symbol Interference (ISI) between M- sequence and OFDM data blocks due to multipath delay. As an example, when 64 subcarriers are used there may be 1,153 blocks in each such OFDM frame.

[0091] M-sequence is widely used in time synchronization because of its high autocorrelation properties. However, when either the transmitter or receiver is in motion, the varying distance and location over time result in dynamic communication links characterized by multipath delay, latency, and Doppler effects. Under these conditions, time synchronization based on the auto-correlation of the M-sequence becomes unreliable. The constant changes in distance and location introduce variability in the signal propagation path, causing fluctuations in the magnitude and phase of the received signal. Additionally, Doppler effects alter the frequency of the received signal, leading to mismatches in the expected signal pattern used for autocorrelation. Consequently, these factors collectively degrade the signal detection accuracy in dynamic environments, limiting the M-sequence to provide only rough time synchronization.

[0092] Therefore, to realize accurate time synchronization, various embodiments provide a synchronization strategy that utilizes both the M-sequence and power detection, as will now be described with respect to FIG.11.

[0093] FIG.11 depicts a flow diagram of a synchronization method according to an embodiment and suitable for implementing a synchronization strategy at a receiver such as described herein and above with respect to the various embodiments. Specifically, the method 100 of FIG.11 is suitable for use in implementing a time synchronization function prior to a channel estimation function at a receiver, such as within the channel estimation and equalization function of the OSDMA demodulator 340 of FIG.3 or 450 of FIG.4.

[0094] At step 1110, the synchronization strategy method performs pseudo-noise (illustratively, M-sequence) autocorrelation; namely, calculating the autocorrelation between the M-sequence and the received signal to obtain a rough estimate of the position of the received M-sequence, and then remove it.

[0095] At step 1120, the synchronization strategy method performs power detection; namely, conducting power detection to verify if the tail of the M-sequence has been successfully removed. Then, removing the zero-GI preceding the OFDM data blocks.

[0096] At step 1130, the synchronization strategy method verifies zero-GI removal; namely, perform power detection again to ensure the tail of the zero-GI has been successfully removed.

[0097] At step 1140, the synchronization strategy method performs OFDM CP autocorrelation; namely, performing auto-correlation on the OFDM CP to achieve precise time synchronization.

[0098] FIG.12 schematically depicts an OSDMA system according to an embodiment. Specifically, the exemplary OSDMA system 1200 of FIG.12 is substantially the same as the OSDMA system 300 of FIG.3. The system 1200 of FIG.12 further illustrates the use of a time synchronization processing module or function 341 within the demodulator 340 that is configured to perform steps 1110, 1120, and 1130 described above with respect to FIG.11.

[0099] Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings. Thus, while the foregoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof.

Claims

What is claimed is:

1. A method of communication, comprising: channel coding and interleaving each of a plurality of input data streams to provide respective sequences of channel coded bitstreams; Spatially Modulating (SM) and mapping to a constellation each of the respective sequences of channel coded bitstreams to provide respective sequences of symbol streams ^^^; dividing each of the sequences of symbol streams ^^^into ^^^blocks by serial-to- parallel conversion; modulating each of the ^^^symbol stream blocks and a respective pilot sequence in accordance with Orthogonal Signal Division Multiplexing (OSDM) to provide thereby respective OSDM signals; and transmitting the OSDM signals via a transmitting device during respective time slots.

2. The communication method of claim 1, wherein the communication method comprises an undersea communication method, and each respective transmitting device comprise a submerged acoustic output device.

3. The communication method of claim 1, wherein the communication method comprises a Radio Frequency (RF) communication method, and each respective transmitting device comprises a RF antenna.

4. The method of claim 1, further comprising: receiving, via each of a plurality of acoustic input devices, some or all of the transmitted OSDM signals; channel estimating received OSDM signals using pilot sequences included therein; channel equalizing and OSDM demodulating received OSDM signals to provide a plurality of channel equalized symbol stream blocks; combining respective ^^^blocks by parallel-to-serial conversion into sequences of symbol streams ^^^; spatially demodulating and constellation de-mapping the channel equalized received sequences of symbol streams s^to retrieve therefrom respective sequences of channel coded bitstreams; andde-interleaving and channel decoding the retrieved sequences of channel coded bitstreams to obtain therefrom respective output bit sequences.

5. The method of claim 4, further comprising, prior to channel estimating the received OSDM signals: correlating received M-sequence data within the received OSDM signals to estimate positions therein of the received M-sequence data; and correlating received cyclic prefix data within the received OSDM signals to enable time synchronization of the received OSDM signals.

6. The method of claim 5, wherein the received OSDM signals comprise Orthogonal Frequency Division Multiple Access (OFDMA) encoded data signals.

7. A communications system, comprising: a transmitter, configured for generating an Orthogonal Signal Division Multiple Access (OSDMA) signal for transmission, the OSDMA transmitter comprising: a channel coding and interleaving module for channel coding and interleaving each of a plurality of input data streams to provide respective sequences of channel coded bitstreams; a Spatial Modulating (SM) and mapping module, for spatially modulating and mapping to a constellation each of the respective sequences of channel coded bitstreams to provide respective sequences of symbol streams ^^^; a symbol dividing module, for dividing each of the sequences of symbol streams ^^^into ^^^blocks by serial-to-parallel conversion; a pilot sequence insertion module, for associated a respective pilot sequence with each member of a group of related symbol stream blocks ^^^; an Orthogonal Signal Division Multiplexing (OSDM) module, for modulating each of the ^^^symbol stream blocks and respective pilot sequences in accordance with Orthogonal Signal Division Multiplexing (OSDM) to provide thereby respective OSDM signals to provide respective OSDM signals suitable for transmission via respective transmitting devices during respective time slots.

8. The communications system of claim 7, wherein the communications apparatus comprises an undersea communications apparatus, and each respective transmitting device comprise a submerged acoustic output device.

9. The communications system of claim 7, further comprising: a receiver, configured for receiving the OSDM signals transmitted via respective transmitting devices during respective time slots, the OSDM receiver comprising: a plurality of acoustic input devices, each acoustic input device configured to receive some or all of the transmitted OSDM signals; and an OSDM demodulator operatively coupled to the acoustic input devices and configured to process the OSDM signals received therefrom in accordance with the steps of: channel estimating received OSDM signals using pilot sequences included therein; channel equalizing and OSDM demodulating received OSDM signals to provide a plurality of channel equalized symbol stream blocks; combining respective ^^^blocks by parallel-to-serial conversion into sequences of symbol streams ^^^; spatially demodulating and constellation de-mapping the channel equalized received sequences of symbol streams s^to retrieve therefrom respective sequences of channel coded bitstreams; and de-interleaving and channel decoding the retrieved sequences of channel coded bitstreams to obtain therefrom respective output bit sequences.

10. The communications system of claim 9, wherein the OSDM demodulator is further configured to process the received OSDM signals in accordance with: prior to channel estimating the received OSDM signals: correlating received M-sequence data within the received OSDM signals to estimate positions therein of the received M-sequence data; and correlating received cyclic prefix data within the received OSDM signals to enable time synchronization of the received OSDM signals.

11. The communications system of claim 10, wherein the received OSDM signals comprise Orthogonal Frequency Division Multiple Access (OFDMA) encoded data signals.

12. An Orthogonal Signal Division Multiple Access (OSDMA) receiver configured for receiving encoded data signals transmitted via a plurality of time slots, the OSDM receiver comprising: a plurality of acoustic input devices, each acoustic input device configured to receive some or all of the encoded data signals; and an OSDM demodulator operatively coupled to the acoustic input devices and configured to process the encoded data signals received therefrom in accordance with the steps of: channel estimating received encoded data signals using pilot sequences included therein; channel equalizing and OSDM demodulating received encoded data signals to provide a plurality of channel equalized symbol stream blocks; combining respective ^^^blocks by parallel-to-serial conversion into sequences of symbol streams ^^^; spatially demodulating and constellation de-mapping the channel equalized received sequences of symbol streams s^to retrieve therefrom respective sequences of channel coded bitstreams; and de-interleaving and channel decoding the retrieved sequences of channel coded bitstreams to obtain therefrom respective output bit sequences.

13. The OSDM receiver of claim 12, wherein the OSDM demodulator is further configured to process the received encoded data signals in accordance with: prior to channel estimating the received encoded data signals: correlating received M-sequence data within the received encoded data signals to estimate positions therein of the received M-sequence data; and correlating received cyclic prefix data within the received encoded data signals to enable time synchronization of the received encoded data signals.

14. The OSDM receiver of claim 12, wherein the received encoded data signals comprise Orthogonal Frequency Division Multiple Access (OFDMA) encoded data signals.

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